This invention relates to a selective anionic ring opening process which comprises reacting episulfides with thiols in the presence of tertiary amines. More specifically, this invention relates to polythioether dithiols and polythiols which are derived by reacting episulfides according to the process of this invention with dithiols and polythiols, respectively.
Several anionic ring opening reactions of episulfides are known in the prior art. For example, U.S. Pat. Nos. 2,490,984 and 2,497,100 describe the nonselective anionic ring opening of episulfides by thiols in the presence of sodium thiolates as catalysts, e.g. ##STR1## wherein R is an alkyl group.
The anionic ring opening of episulfides by primary and secondary amines is also known. The reaction was reviewed in some detail by M. Sander (see Chemical Reviews, 66, 331-333, (1966)). For example, A. Oddon and J. Wylde reported that secondary amines selectively open the propylene episulfide ring via an anionic mechanism to form the corresponding secondary aminoethanethiols; e.g., ##STR2## (For reference see Bull Soc. Chim., France, pp. 1603-1607, 1967).
The ring opening of episulfides by thiols in the absence of catalysts is known (see German Pat. No. 696,774). However, such reactions require higher temperatures.
It is believed that the present tertiary aliphatic amine catalyzed ring opening of episulfides by thiols is most closely related to a similar ring opening of epoxides. In contrast to the products of this invention, the epoxide ring opening products are, however, generally discolored as a result of the formation of colored by-products.
Many polythioethers derived from episulfides by anionic ring opening reaction are also known. P. Sigwalt in a review article (P. Sigwalt, "Ring Opening Polymerization", 2, 191-217 (1969) in a series of monographs on "Kinetics and Mechanisms of Polymerization", Ed. K. C. Frisch, M. Dekker, New York, N.Y.) discusses the various catalysts, among them sodium thiolates and tertiary amines, which have been used to effect anionic episulfide polymerizations. (For reference also see Chimie Industrie-Genie Chimique, 96, 909, (1966) and S. Adamek, B. B. J. Wood and R. T. Woodhams, Rubber and Plastics Age, 56 (1965)).
Anionic ring opening reactions of episulfides with compounds of metals of Group Ia of the Mendeleef Periodic Table in the presence of a compound containing a labile hydrogen atom as a chain transfer agent are known to the art; see for example, South African Application No. 65/1838.
In recent years a number of patents covering various sulfur containing polymeric compositions have issued, e.g., U.S. Pat. Nos. 3,403,187; 3,432,542; 3,465,064; 3,466,336; 3,484,418; 3,503,940; 3,504,050; 3,544,543; 3,625,925; 3,644,302 and 3,686,326. These patents do not, however, teach, suggest or disclose polythioether thiol compositions having the stereoregular feature, i.e., units of the same orientation relative to the thiol group. ##STR3## These patents do not teach that certain tertiary amine and tertiary phosphine catalysts will selectively cause a ring opening of the episulfide in the presence of thiols to obtain the aforementioned stereoregular polythioether thiols.
British Patent Specification No. 1,082,565 to Cameron discloses a process for preparing "Flexibiliser C" wherein hydrogen sulfide is reacted with propylene sulfide in the presence of N-benzyldimethylamine at 60.degree. C. Cameron states at page 2, lines 62-74:
"The aforesaid flexibilisers, many of which are known compounds, may readily be obtained from the monoepisulfide and the compound containing at least two hydrogen atoms as aforesaid by reaction in the presence of an acid catalyst, such as boron trifluoride diethyl etherate, or a basic catalyst such as sodium ethoxide or N-benzyldimethylamine. If the compound used to react with the monoepisulfide is an amine, addition of a catalyst is usually unnecessary." Among the compounds containing at least two hydrogen atoms attached directly to nitrogen or sulfur which may be used to form the flexibilizing agents include hydrogen sulfide and polythiols. The Cameron specification does not teach the combination of using more than one episulfide or the importance of selecting certain tertiary amines or phosphines for preparing polymers having stereoregularity.
Copolymers of two or more episulfides are also known to be formed by anionic reactions, as it is disclosed in U.S. Pat. No. 3,317,920. Most of such copolymers were prepared to incorporate minor amounts of olefinic episulfides whose double bond could be subsequently utilized for the crosslinking of the resulting polythioethers by conventional sulfur vulcanization methods. In all these preparations the copolymerization of various episulfides occurred in a statistically random fashion.